Direct-viewing half-tone storage tube



July 21, 1970 KISAO TAKAYAMA ET A 3,521,118

DIRECT-VIEWING HALF-TONE STORAGE TUBE Filed March 15, 1965 FIG. 2

FIG. 1

United States Patent 3,521,118 DIRECT-VIEWING HALF-TONE STORAGE TUBE Hisao Takayama, Kozo Natsume, Shigeru Minakami, and

Mitsuharu Hoshi, Tokyo, Japan, assignors t0 Nippon Aviotronics Company Limited, Tokyo, Japan, a corporation of Japan Filed Mar. 15. 1965, Ser. No. 439,656 Claims priority, application Japan, Mar. 16, 1964,

39/ 14,324; Dec. 22, 1964, 39/ 80,625

Int. Cl. H01j 9/12 US. Cl. 31512 1 Claim This invention relates to direct-viewing half-tone storage tubes and more particularly to an improved structure of storage mesh electrode for such tubes and a method for making such electrodes for use herein that are capable of providing an increased writing speed and viewing duration and further to a method for operating directviewing half-tone storage tubes.

One of the important characteristics of the direct-viewing half-tone storage tube is the maximum writing speed. It has been deemed impossible to attain the maximum writing speed of 1x10 cm./sec. or more in those heretofore produced by prior art electrodes of this type. However, in the case of utilizing the direct-viewing storage tube as a cathode ray tube of oscilloscope for observing an instantaneous Wave form, there are many cases, such as observation of single pulse in an electric circuit, discharging, various wave forms and the like, that requires the maximum writing speed of 1 10 cm./sec. or more. Therefore, it has been an important problem to increase the maximum writing speed for the direct-viewing storage tubes. There are many factors in designing tubes of this type as hereinafter described that have an effect on the maximum writing speed of half-tone storage tubes, however, none of these factors has been effective for increasing the maximum writing speed as compared to a value heretofore available on account of several reasons.

It is therefore an object of this invention to provide a storage mesh electrode for direct-viewing half-tone storage tubes that affords increasing of the maximum writing speed by way of increasing a thickness of storage layer disposed over an electroformed mesh.

Another object of this invention is to provide an improved method of operation for preventing the decrease of viewing duration causing from the increase of the maximum writing speed.

The novel features which are believed to be characteristic of the invention, both as to its organization and method of operation, together with further objects and advantages thereof, will be better understood from the following description considered in connection with the accompanying drawings in which;

FIG. 1 is a sectional view of the direct-viewing halftone storage tube of the present invention; and

FIG. 2 is a view of enlarged scale of portion in a process for fabricating the storage mesh electrode.

Referring now to the drawings, FIG. 1 shows a sectional view of an embodiment of the direct-viewing halftone storage tube of the present invention. An evacuated glass envelope 18 consists of a face portion 1, a neck portion 17 and an enlarged cylindrical or cone portion ice 10. An electron gun 16 and an electron gun 15 are disposed in the neck portion 17 of the envelope. A storage mesh electrode assembly 5, 8 and a cylindrical electrode 9 are disposed adjacent to and coextensive with the face portion 1 at the end of the cone portion 10.

Along the mesh screen 5 are disposed, as it is illustrated in FIG. 3, storage layers 6, 7 and a conductive film 4. A fluorescent screen 2 is disposed on the inner surface of the face portion 1 and a conductive film 3 is disposed thereon. A cylindrical graphite electrode 11 is disposed concentrically about the inner periphery of the cone portion 10. The mesh of screen 5 is called a storage mesh and that the mesh of screen 8 is a collector mesh. The mesh of screen consists, for example, of an electroformed nickel mesh having of the order of lines/cm. and a transparency of 60%. The electron gun 16 is called a writing gun, which has the same structure as those utilized in conventional cathode ray tubes of oscilloscopes and produces a high energy and high density narrow electron beam 19. The necessity of electron beam 20 is to arrive at the storage mesh assembly 5, 8 with uniform density over the entire area thereof together with uniform and normal incidence thereto. The cylindrical electrode 9 constitutes a collimating electron lens and is used for collimation of the charging electron beam 20. In the operation, the storage layer 7 is scanned by the electron beam 19 produced by the writing gun 16, while charging gun 15 is also operating. This, then, produces a charged pattern on the storage layer 7 in accordance with the scanning and is stored. The cone shaped electron beam 20 from the charging gun 15 is accelerated through interstices or holes in the collector mesh 8 and the storage mesh 5, by a high potential impressed between the conductive film 3 and the storage mesh 5, to strike the fluorescent screen 2, thereby causing luminescence.

Since the flow of charging beam 20 is modulated by work of a field distribution formed by the charged pattern stored on the storage layer 7 when passing through the interstices of the storage mesh 5, the luminescence appears only on areas of the fluorescent screen 2 corresponding to the charged pattern on the storage layer 7. Thus, a desired scanning signal is stored on the storage layer and read out simultaneously therefrom.

In the preferable mode of operating the storage tube, cathode 14 of the charging gun 15 is maintained at the ground potential or zero volt, cathode 21 of the Writing gun 16 is maintained 3,000 volts with respect to ground, anode 22 of the writing gun 16 is maintained +100 volts with respect to ground, anode 12 of the charging gun 15 is maintained +40 volts with respect to ground, the storage mesh 5 is maintained from +2 to +15 volts with respect to ground, the collector mesh 8 is maintained +60 volts with respect to ground and the conductive film 3 is maintained +8,000 volts.

Now assuming that the maximum value of electron current of the writing gun is i (Ampere), a diameter of the electron beam at the surface of storage layer is b (mm), the thickness of the storage layer is d(,u.), dielectric constant is e, secondary emission ratio is 6 and change of the surface potential of the storage layer that requires to vary screen brightness from 0 to 100% is AV the maximum writing speed W.S. (cm./ sec.) of the writing electron beam, which is recordable, in the direct-viewing halftone storage tube may be given by the following equation;

41rk.d.i.(5-l) EbAVn) (cur/sec.)

wherein k is a constant. To increase maximum writing speed, as it is apparent from the equation, AV should be decreased, as by changing the structure of the storage mesh. Experience shows that it is difficult to decrease AV on a large scale. Increase of the maximum electron beam current i of the writing gun results in the increase of diameter b of the electron beam at the storage surface, this, in turn, results in a sacrifice of resolution. Considerable increase of the maximum writing speed W.S. cannot be attained in this manner. Further, due to the limitations in selecting of dielectric material for the storage layer, to increase the secondary emission ratio 6 of the storage layer cannot be increased on a large scale. Furthermore, the dielectric constant e is limited to the minimum of the order of 1 and the thickness of storage layer heretofore designed is a value of 11.5,u. It would be possible to increase the maximum writing speed by way of increasing the thickness of storage layer. Such increase of the thickness of storage layer has not been deemed practical heretofore. Difficulty is encountered in fabricating the storage mesh of increased thickness.

An excess of thickness of the layer of magnesium fluoride, which material is normally used as layer of storage electrode for direct-viewing half-tone storage tubes, over the aforementioned designed value of ll.5u results in a sacrifice of quality of the storage layer and there may be a risk of falling off. A second reason consists in the decrease of viewing duration.

In accordance with the present invention, such difliculties described above can be eliminated as hereinafter described in detail and it is possible to increase the maximum writing speed of the direct-viewing half-tone storage tube as ten times as much as that available with the prior art storage layers of this type by increasing the thickness, to more than 10,14, of the storage layer.

The first principal step in thickening the storage layer of the present invention is to select another storage material such as, for example, calcium fluoride. Calcium fluoride can easily be applied thicker than magnesium fluoride, however, if evaporation of calcium fluoride is effected in a pressure at or above the order of 1X10 mm. Hg to attain the layer having the thickness of -40,u, such layer of calcium fluoride may clog the interstices of the storage mesh 5. This action results in a loss of transparency of the storage mesh 5, lowering the brightness of the stored wave form and causing an unstable storage action. On the other hand, with evaporation of the material for the storage layer at a pressure at or below 3X10- mm. Hg, it is found that a layer having a good transparency is obtained and the brightness is high enough.

To further increase the maximum writing speed, the secondary emission ratio of the storage layer, is increased. According to an embodiment of the invention, on the layer of calcium fluoride (storage layer 6), which is excellent in depositing characteristic by evaporation, a thin layer of magnesium fluoride (storage layer 7) having high secondary emission characteristic and a thickness of the order of 23,u. is applied by evaporation.

It has been found that the characteristics of the half-tone storage tube is lost when the thickness of stor age layer is increased more than approximately the order of 40a, and that the tube will exhibit bi-stable type characteristic together with decrease of the maximum writing speed. Therefore the thickness of storage layer of the storage tube of this invention is preferably kept between from 10 to 40p. thick.

In the operation of the direct-viewing half-tone storage screen and loss of the contrast in the visual image. This duration is called the viewing duration and which duration is shortened in reverse proportion to the thickness of storage layer. In conventional direct-viewing half-tone storage tubes, this viewing duration is the order of 60-300 seconds; it is undesirable to shorten the viewing duration from this order for practical usage.

According to this invention, since the thickness of storage layers 6, 7 are increased by ten times or more as compared to those heretofore produced, the viewing duration should theoretically be shortened to one-tenth of those available with the prior art storage layers of this type. However, in the preferred embodiment of this invention, this shortage of viewing duration is compensated by a novel method. There are several ways to compensate such shortage of the viewing duration. One method in which the compensation may be performed is to utilize a viewing duration enhance circuit for decreasing an amount of viewing electron beam 20 equivalently in such a manner as emitting the viewing electron beam 20 intermittently and decreasing an amount of ions to be produced in the storage tube, thereby substantially extending the viewing duration. This method, however, requires an extra electric circuit and the expense therefor for operating the viewing electron beam in the above said manner. Further, in this method, there may be such disadvantage that differentiated pulses of the intermittent wave form given to a first grid 13 of the charging gun are transmitted to a cross deflection plate 23 of the writing gun through a static capacitance between the first grid 13 and the deflection plate 23 of the writing gun. These differentiated pulses are then superposed on a signal wave form for causing a disturbance therein. According to this invention, a uniform, low velocity and low density viewing electron beam 20 is formed by decreasing a voltage of anode 12 of the charging gun from a normal voltage of v. to approximately 40 v. and bringing a biasing voltage of the firse grid 13 of the flood gun up to the vicinity of 0 volt. Thus, an amount of ions to be produced in the storage tube is considerably decreased and the viewing duration is made to be equal to that heretofore available with the prior art direct-viewing half-tone storage tubes.

Further, in accordance with this invention, it is possible to produce the visual image of the correct signal without any disturbance together with decreasing so-called uneven brightness effect in a stored condition owing to the fact that the viewing electron beam 20 spreads uniformly because of low accelerating voltage and biasing voltage.

In the preferred embodiment of this invention, by utilizing such storage electrode comprising the mesh electrode having the order of 10.0 lines/cm. and the light transparency of 60%, which is disposed, along the meshes of electrode, the 20 thick layer of calcium fluoride, the 2g thick layer of magnesium fluoride thereon and the la thick layer of aluminum on the other side thereof by evaporation, under such mode of operation as described hereinabove and with the writing speed of 2 X10 cm./ sec. or more and with the viewing duration of 100 seconds or more, a stable and definite wave form having uniform screen brightness can be obtained on the viewing screen. This is found to be a satisfactory operation of the directviewing half-tone storage tube of the present invention. Therefore, the storage mesh electrode according to this invention is improved considerably as compared to the prior art storage electrodes. According to this invention, however, the brightness of the fluorescent screen 2 is slightly decreased due to the decrease of an amount of the viewing electron beam incident on the screen 2 is not a disadvantage, however, because the brightness of the direct-viewing half-tone storage tube is bright enough and, if necessary, a desired brightness can easily be attained by increasing the voltage of the conductive layer 3.

While there has been described herein the invention in connection with a specific embodiment, it is to be clearly 5 6 understood that this description is made only by way of the grid voltage of said charging gun being maintained example and not as a limitation to the scope of this inat -10 volts or above with respect to the cathode vention. electrode of said charging gun. What is claimed is: 1. A direct-viewing half-tone storage tube comprising: References Clted a storage mesh having a storage layer thereon, said 5 UNITED STATES PATENTS storage layer having an overall thickness of between 10 and 40,11 and being comprised of a layer of calcium 52:? fluoride and a layer of magnesium fluoride having a thickness of about 23,U. applied over said layer of 2790929 4/1957 Herman et 313*68 XR 10 2,856,559 10/1958 Knoll 315-12 calcium fluoride; a charging gun having grid and cathode electrodes; the accelerating voltage of said charging gun being RICHARD A FARLEY Primary Examinm.

maintained at approximately 50 volts or below with respect to the cathode electrode of said charging gun; 15 MORRIS, Asslstant EXaInlIlel" and 2,962,623 11/1960 Beinterna 315-12 

1. A DIRECT-VIEWING HALF-TONE STORAGE TUBE COMPRISING: A STORAGE MESH HAVING A STORAGE LAYER THEREON, SAID STORAGE LAYER HAVING AN OVERALL THICKNESS OF BETWEEN 10 AND 40$ AND BEING COMPRISED OF A LAYER OF CALCIUM FLUORIDE AND A LAYER OF MAGNESIUM FLUORIDE HAVING A THICKNESS OF ABOUT 2-3$ APPLIED OVER SAID LAYER OF CALCIUM FLUORIDE; A CHARGING GUN HAVING GRID AND CATHODE ELECTRODES; THE ACCELERATING VOLTAGE OF SAID CHARGING GUN BEING MAINTAINED AT APPROXIMATELY 50 VOLTS OR BELOW WITH RESPECT TO THE CATHODE ELECTRODE OF SAID CHARGING GUN; AND THE GRID VOLTAGE OF SAID CHARGING GUN BEING MAINTAINED AT -10 VOLTS ARE ABOVE WITH RESPECT TO THE CATHODE ELECTRODE OF SAID CHARGING GUN. 